(a) Field of the Invention
This invention relates to an improvement in or relating to a capacitance sensor which is adapted to detect a variety of information by making use of variable capacitance transducers.
(b) Description of the Prior Art
Variable capacitance transducers, which may be considered as a sort of capacitor which is capable of converting some physical or chemical parameter or quantity into a capacitance of a magnitude corresponding to the physical or chemical parameter or quantity, have heretofore been employed, for example, to detect, externally of a an intravenous bottle without making contact with the liquid in the bottle per se, that the level of the liquid has reached a predetermined level. Referring now to FIG. 1 which is a simplified schematic block diagram of a conventional capacitance sensor, the prior art liquid level detection system will be described. On suitable locations of the outer wall of a bottle 10, there are applied in a face-to-face relationship, two electrodes 11, 12 which constitute a transducer. As soon as the level of the liquid 14 within the bottle 10 has fallen down below the electrodes 11, 12, this is detected by sensing a capacitance change between the electrodes 11, 12, which capacitance change has occurred due to a descent of the level of the liquid 14 below the electrodes 11, 12, by a capacitance change detecting circuit 15. The operation of the above conventional capacitance sensor will next be described with reference to FIG. 2. Variations in capacitance of the transducer are diagrammatically illustrated by a solid line 20 in FIG. 2, as a function of the level of the liquid. The capacitance of the transducer becomes maximum C.sub.max when the interspace between the electrodes 11 and 12 is filled liquid with the drops. When the drops filling liquid the interspace between the electrodes 11 and 12 is replaced by air, the capacitance of the transducer reaches the minimum level, C.sub.min. Thus, by setting the detectable threshold level of the capacitance change detecting circuit 15 at a value somewhat higher than the minimum value C.sub.min, it is possible to carry out the accurate detections of liquid levels at the same detectable threshold level even if the liquid 14 is replaced by other liquid of different dielectric constant .epsilon. and the capacitance variations of the other liquid is represented by, for example, broken lines 22, 23 in FIG. 2, to say nothing of a liquid 14 represented by the solid line 20.
The minimum value C.sub.min of the interelectrode capacitance varies with environmental conditions for measurement such as the type of bottle used and the temperature. Thus, if the detectable threshold level is set too low, the minimum value C.sub.min may in some instance be greater than the threshold level, thereby making the capacitance sensor fail to work. To avoid such a potential danger, it has heretofore been required to reset the threshold level whenever a fresh liquid-containing drop bottle is used, leading to a drawback that the handling of the capacitance sensor is cumbersome.
It has generally been known to employ a circuit which makes use of the AC capacitance bridge method or LC resonance method, as the capacitance change detecting circuit 15. These prior art circuits are however accompanied by drawbacks in that they render devices or instruments unavoidably voluminous and expensive, because the former method requires a complex structure and a stable oscillator and a detection circuit is indispensable for the latter method. It has been proposed, as a countermeasure for the above problems, to employ another capacitance change detecting circuit of a relatively simple structure, in which the oscillation circuit is formed using the capacitance of a transducer as a feedback capacitor so as to detect each capacitance change on the basis of a change in oscillation output. This capacitance change detecting circuit is however accompanied by another drawback in that its use is limited to materials or items which exhibit a large change in capacitance, because it is difficult to control the oscillation output by a small degree of capacitance change. Thus, when a capacitance change detecting circuit of the above type is applied to detect the liquid level of a liquid containing drop bottle, there is a problem in that, for example, a communicating vessel of a small diameter has to be provided with the bottle and it is necessary to put a transducer on the thus-provided communicating vessel since, if its electrodes are provided on the bottle per se as illustrated in FIG. 1, the interelectrode spacing will be too wide to detect a small change in capacitance.